Fixed a bunch of problems when moving among agenda/day/week/month views.

Fixed a bunch of problems when moving among agenda/day/week/month views.

b/1438315 Fixed problems introduced in the new back behavior.
b/2077287 Fixed the problem where you sometimes end up in 1969 when switch among the agenda/day/week/month views.
b/2076791  The selected date is preserved when switch to various views.
Removed scroll bar in agenda view as the header was not a static list of events anymore
Cleaned up some TODOs.